Sudoku

Background

Despite its name, Sudoku first appeared in the United States, under the name Number Place, in 1979 (although a similar puzzle ran in various French periodicals from the late 1880s to the 1910s.

It was then introduced by Nikoli to a Japanese audience in 1984, later adding the restriction that clues be symmetrical (usually rotationally), as well as limiting the clues to 32. A common Number Place restriction, that no numbers be repeated in the long diagonals, was absent in Nikoli's rules, although it later became a variant (Sudoku X).

The original full name in Japanese was <em>Sūji wa dokushin ni kagiru,</em> 数字は独身に限る, that is, "The digits are limited to one occurrence"; "sudoku" is an abbreviation of the kanji.

The name "sudoku" has become so ubiquitous that Dell Magazines (original publishers of Number Place) has embraced it.

Puzzle and Goal

An unsolved puzzle consists of a 9x9 grid containing nine 3x3 regions. Some of the cells contain a number between 1 and 9.

The goal is to fill in all the cells so as to create a Latin Square.

Rules

The solved grid must satisfy the following conditions:

Every rank and region must contain each digit from 1 to 9 (or, in variants, 1 to n, where n is the size of the grid).

Variants

There are a dizzying number of variants to sudoku, some of which have separate entries in this site.

6x6 grids are easier, and are often provided for warm-up or beginners; 12x12, 16x16, and 25x25 also appear.

Puzzles might overlap, so that one central sudoku shares each corner nonet with another sudoku, for a total of five ("Samurai Sudoku").

Additional restrictions might be applied, such as only allowing odd or even numbers in shaded cells ("Odd Even Sudoku"), using letters instead of words and requiring certain cages contain words from a provided list, placing inequality symbols on certain edges such that the cell values render it true ("Comparison Sudoku"), having additional cages that must contain unique numbers ("Hyper Sudoku", e.g.), and so on.

It appears that the enduring popularity of sudoku lies in the twin features that its core rules are very simple and it provides a great deal of flexibility of variation.